2. Refrigeration defined as the process of achieving and maintaining a
temperature below that of the surroundings, the aim being to cool
some product or space to the required temperature. In other words,
refrigeration is artificial (human-made) cooling. Energy in the form of
heat is removed from a low-temperature reservoir and transferred to a
high-temperature reservoir. The work of energy transfer is traditionally
driven by mechanical means, but can also be driven by
heat,magnetism,electricity,laser, or other means.
4. ➢ Domestic refrigeration is rather limited in scope, being concerned primarily
with household refrigerators and home freezers.However,because the number
of units in service is quite large, domestic refrigeration represents a significant
portion of the refrigeration industry.
5. Commercial refrigeration is concerned with the designing, installation, and
maintenance of refrigerator used by retail stores,restaurants, hotels and
institutions for storing and dispensing of perishable commodities of all type.
6. Transportation refrigeration is concerned with refrigeration equipment as it
applied to trucks, both long transports and local delivery,and to refrigerated
rail way cars.
7. • Industrial refrigeration is often confused with commercial refrigeration because
the division between these two areas is not clearly defined.
• Industrial refrigeration is larger in size than that of commercial and has the
distinguishing feature of requiring an attendant on duty, usually a licensed
operating engineer.
• Typical industrial application are ice plants, large food packing plants, creameries,
and industrial plants.
8. ➢ Marine refrigeration refers to aboard marine vessels and includes, for example
refrigeration for fishing boats and for vessel transporting perishable cargo as
well as refrigeration for the ships stores on vessels of all kinds.
9.
10. • The preservation of perishables by refrigeration involves the use of low
temperature as mean of eliminating or retrading the activity of spoilage
agents .
• The storage of perishable at low temperatures greatly reduce the activity of
both enzymes and micro-organsims and thereby provides a practical means
of preserving perishables in their original fresh state for varing periods of
time.
12. In olden days refrigeration was achieved by natural means such as the use
of ice or evaporative cooling.
In Europe, America and Iran a number of icehouses were built to store ice.
Materials like sawdust or wood shavings were used as insulating materials in these
icehouses. Later on, cork was used as insulating material.
In earlier times, ice was either:
1.Transported from colder regions,
2. Harvested in winter and stored in ice houses for summer use or,
3. Made during night by cooling of water by radiation to stratosphere.
14. The ice was stored inside the ice house in a pit like this,and the surrounding earth and
layers of straw would help insulate the ice.
15. The dome of the ice house helped keep the ice from melting,and at the top there was a hole
covered with a piece marble that prevented the cool air from exiting but allowed daylight to enter.
16. The ice house at Moggerhanger Park,
Moggerhanger,Bedfordshire
17. The ice house entrance, Eglinton
Country Park,Scotland
19. In this method ice was made by keeping a thin layer of water in a shallow
earthen tray, and then exposing the tray to the night sky. Compacted hay of
about 0.3 m thickness was used as insulation.The water looses heat by
radiation to the stratosphere, which is at around -55°C and by early morning
hours the water in the trays freezes to ice.This method of ice production was
very popular in India.
20. Evaporative cooling is the process of reducing the temperature of a system by
evaporation of water. Evaporative cooling has been used in India for centuries to
obtain cold water in summer by storing the water in earthen pots.The water
permeates through the pores of earthen vessel to its outer surface where it
evaporates to the surrounding, absorbing its latent heat in part from the vessel,
which cools the water.
21.
22.
23. :
Certain substances such as common salt, when added to water dissolve in
water and absorb its heat of solution from water (endothermic process). This
reduces the temperature of the solution (water+salt). Sodium Chloride salt
(NaCl) can yield temperatures up to -20°C and Calcium Chloride (CaCl2) up
to - 50°C in properly insulated containers. However, as it is this process has
limited application, as the dissolved salt has to be recovered from its solution
by heating.
24. ❖
.
is a cyclic device & the working fluid used in the
refrigeration cyclesare called refrigerants.
❖ Another device that transfer heat from a low- temperature medium
to a high- temperature one is the heat pump. Refrigerators & heat
pumps are the same devices they differ in their objectivesonly.
25. As shown in figure bellow the Q1 is the magnitude of the heat removed from the
refrigerated space at temperature T1 and Q2 is the magnitude of the heat rejected to the
warm space at temperature T2 and W net is the net work input to the refrigerator.
26. The objective of refrigerator is to maintain the refrigerated space at low
temperature by removing heat from it.
The objective of a heat pump is to maintain a heated space at a high
temperature.
This is accomplished by absorbing heat from a low- temperature source such as
cold outside air in winter & supplying this heat to a warmer medium such as a
house.
27. The performance of refrigerators & heat pump is expressed in terms of the
coefficient of performance (COP)
28. The cooling capacity of refrigeration systemwhich is the rate of heat removal
from the refrigeratedspace is often expressed in termsof Ton of Refrigeration
(TR).
29. Example 1 : Find the C.O.P. of a refrigeration system if the
work input is 80 kJ/kg and the refrigeration effect
produced is 160 kJ/kg of refrigerant flowing.
Solution :
Given w = 80kJ/kg, q= 160 kJ/kg
COP=q/w =160/80 =2
30. The heat engine that operates most efficiently between high-temperature reservoir& a
low- temperature reservoir is the Carnot engine. It is an ideal engine that uses reversible
processes to form its cycle of operation. Thus it is also called a reversible engine
31. We can see that the thermal efficiency
of a Carnot engine is dependent only
on the high & low absolute
temperature of the reservoirs.
This relation of efficiency is applicable
for all working substances or for all
reversible engines regardless of the
particular design characteristics.
32. The Carnot engine when operated in reverse become a heat pump or refrigerator.
Depending on the desired heat transfer. The COP for a heat pump becomes:
The COP of refrigerator becomes:
The a above measures of performance set limits that real devices can only approach. The
reversible cycles assumed are unrealistic but the fact that we have limits which we know
we cannot exceed is often very helpful in evaluating proposed design & determining the
direction for further effort.
33. Example 1 :
A machine working on a Carnot cycle operates between 305K and 260K Determine the
C.O.P. when it is operated as : (1). refrigerating machine; (2). a heat pump; and
(3). a heat engine.
Example 2:
A cold storage is to be maintained at -5 °C while the surroundings are at 35 °C. the heat
leakage from the surroundings into the cold storage is estimated to be 29kW. The actual
C.O.P. of the refrigeration plant is one-third of an ideal plant working between the same
Temperatures. Find the power required to drive the plant.
34. Is the most widely used cycle for refrigerators,
air- conditioning systems and heat pump. It
consists of four processes:
1-2 isentropic compression in a compressor.
2-3 constant pressure heat rejection in condenser
3-4 throttling in an expansion device
4-1 constant pressure heat absorption in an
evaporator.
35.
36. In an ideal vapor compression
refrigeration cycle. The refrigerant
enters the compressor at state 1 as
saturated vapor & is compressed
isentropically to the condenser
pressure. The temperature of the
refrigerant increases during this
isentropic compression process to
well above the temperature of the
surrounding medium.
37. The refrigerant then enters
the condenser as a
superheated vapor at state 2
and leaves as saturated liquid
at state 3 as a result of heat
rejection to the surroundings.
The temperature of the
refrigerant at this state is still
above the temperature of the
surrounding.
38. The saturated liquid
refrigerant at state3 is
throttled to the evaporator
pressure by passing it through
an expansion valve or capillary
tube. The temperature of
refrigerant drops below the
temperature of the
refrigerated space during this
process.
39. The refrigerant enters the
evaporator at state 4 as low
quality saturated mixture and
it completely evaporates by
absorbing heat from the
refrigerated space. The
refrigerant leaves the
evaporator as saturated vapor
and re enters the compressor,
completing the cycle.
40. The steady flow energy equation per unit mass
The condenser & evaporator do not involve any work and the compressor can be
approximated as adiabatic.
41. An actual vapor compression
cycle differs from the ideal
one in several ways, mostly to
the irreversibility that occur in
various components. Two
common sources of
irreversibility are fluid friction
(causes pressure drop) & heat
transfer to or from the
surrounding.
42. • Sometimes the vapor refrigerant is required to be delivered at a very high pressure.
• In such cases either we should compress the vapor refrigerant by employing single
stage compressor with a very high pressure ratio or compress it in two or more
compressors placed in series.
• The compression carried out in two or more compressors is called compound or
multistage compression.
43. • The COP can be increased either by increasing the refrigerating effect or by
decreasing the compression work. The compression work is greatly reduced if
the refrigerant is compressed very close to the saturated vapor line.
• This can be achieved by compressing the refrigerant in more stages with
intermediate inter-cooling.
44.
45. m1: mass of refrigerant passing through the
evaporator (low pressure Compressor) (kg/min)
m2: mass of refrigerant passing through the
condenser (high pressure Compressor) (kg/min)
m3: mass of liquid evaporated in the inter cooler
m3 = m2 - m1
46. The value of m2 may obtained by considering the thermalequilibrium for the liquid
inter cooler
m2 h5 + m1 h2 = m1 h6 + m 2h3
The mass of liquid evaporated in the intercooler
47.
48. The absorption refrigeration system is one of the oldest method of producing
refrigeration effect. The principle of vapor absorption was first discovery by Michael
faraday in 1824.
This system may be used in both the domestic & large industrial refrigerating plants.
The refrigerants commonly used in a vapour absorption system are ammonia & water.
The vapor absorption system uses heat energy instead of mechanical energy as in
vapor compression system.
In the vapor absorption system, the compressor is replaced by an absorber, a pump, a
generator & a pressure reducing valve.
49.
50. - The refrigerant vapor leaving the evaporator is absorbed in the low temperature hot
solution in the absorber. This process is accompanied by the rejection of heat. The
refrigerant in water solution is pumped to the higher pressure & is heated in the
generator.
- Due to increasing the temperature of solution in generator as a result of adding heat,
most of the refrigerant is evaporated & removed from the solution. The vapor then
passes to the condenser & the weak solution is returned to the absorber.
51.
52.
53.
54.
55. This system uses a solution of lithium bromide in water. In this system, the water is being
used as a refrigerant where as lithium bromide, which is a highly hydroscopic salt, as an
absorbent. The lithium bromide solution has a strong affinity for water vapour because
of its very low vapour pressure .
This system is very popular for air conditioning in which low refrigeration temperature
(not below 0c ) are required.
56. Lithium bromide- water system designed in two forms:
The first where all the components of cycle are placed in same shell, its upper half
contain the generator & condenser while its lower half contain the evaporator &
absorber this type called (one shell system) as in simple absorption system.
The second form consists of two shells, the first shell (high pressure side) contain the
generator & condenser, while the second shell (low pressure side) contain the
evaporator & absorber. This system called (two shell system) as show in figure.
The pressure difference between the generator & the absorber & the gravity due to
the height difference of the two shells is utilized to create the pressure for the spray.
57.
58.
59.
60.
61. A typical refrigeration system consists of several basic components
such as compressors, condensers, expansion devices and
evaporators in addition to several accessories such as filters. It is
essential to study the design and performance characteristics of
individual components.
Is the most important & often the costliest component (typically
30 to 40 percent of total cost) of any vapor compression
refrigeration system. The functions of a compressor is to
continuously draw the refrigerant vapour from the evaporator so
that a low pressure & low temperature can be maintained in the
evaporator at which the refrigerant can boil extracting heat from
the refrigerated space.
62. 1- According to the method of compression
a- Reciprocating compressors
b- Rotary compressors
c- Centrifugal compressors
The compressors may be classified in many ways as follow
72. 5- According to the location of the prime mover:
a- semi- hermetic compressors (direct drive, motor &
compressor in separate housing)
b- hermetic compressors (direct drive, motor &
compressor in same housing.
73. It is the compressor in which the vapor refrigerant is compressed
by the reciprocating motion of the piston. These compressors are
used for refrigerant which have comparatively low volume per kg
& large differential pressure such as ammonia R-717,R-12, R-22.
These compressors are available in small size which are used in
small domestic refrigerators & large size for large capacity
installations.
There are two types of reciprocating compressors in general use
which are single acting vertical compressors & double acting
horizontal compressors.
74.
75. Piston displacement volume or stroke volume:
It is the volume swept by the piston when it moves from its top or
inner dead position to bottom or outer dead center position.
D: diameter of cylinder.
L: length of piston stroke
Clearance factor:
It is the ratio of clearance volume (Vc) to the piston displacement
volume (Vp)
76. It is the volume of the actual amount of refrigerant passing
through the compressor in a unit time it is equal to the suction
volume (Vs) and expressed in (m3/s)
It is the ratio of the compressor capacity or the suction volume (Vs)
to the piston displacement volume (Vp).
77. Suction pressure:
It is the a absolute pressure of refrigerant at the inlet of a
compressor.
Discharge pressure:
It is the absolute pressure of refrigerant at the outlet of a
compressor.
Compression ratio (pressure Ratio):
It is the ratio of absolute discharge pressure to the absolute
suction pressure. Since the absolute discharge pressure is always
more than the absolute suction pressure, therefore the value of
compression ratio is more than unity.
Suction volume:
It is the volume of refrigerant sucked by the compressor during its
suction stroke.
